This invention relates to an electric antenna apparatus carried on vehicles such as automobiles, and more particularly to an antenna apparatus of the motor driven type in which an antenna rod is extended and retracted by a motor by a switch operation.
The motor drive type antenna apparatus of the prior art uses an antenna rod consisting of a plurality of rod members, which are telescopically coupled with one another. The antenna rod is extended and retracted by a motor. This antenna apparatus is constructed so as to provide an upward operation for the extension of the antenna rod as well as a downward operation to retract and receive the antenna rod. It is provided with an operation switch from the generation of the operating instruction. When the switch is set to the upward position, drive power is supplied to the motor and the motor rotates in a first direction to raise the antenna rod. When the switch is set to the downward position in order to retract the antenna rod, on the other hand, the same motor is supplied with drive power of a polarity opposite to that used in the raising operation. This causes the motor to rotate in a second direction, which is opposite to the first direction, thus lowering the antenna rod. When the antenna rod has reached the uppermost position or the lowermost position, the motor is stopped by this switch.
When the antenna rod is at the uppermost and lowermost positions, movement of the motor is impeded and locked. In such an extreme situation, overcurrent flows into the motor, possibly causing burning trouble in the motor. To prevent burning trouble, a torque limiter mechanism has usually been employed in the prior art. However, when the antenna rod is stopped during an up or down operation, the torque limiter mechanism allows the motor per se to slip and rotate.
This mechanism inherently is of a large size. In this respect, the mechanism is undersirable for a motordriven type antenna apparatus, the size reduction of which is preferable and has been demanded. To cope with this, an electrical control means has been used to detect an extreme increase of the load current, which occurs at the uppermost or lowermost position of the antenna. Upon detection, the control means stops the motor current.
FIG. 15 illustrates how up and down movements of the above antenna rod are controlled. As shown, when the switch is turned on, the motor current abruptly rises at the time of start and settles down to a stationary current for driving the antenna rod. In this stationary state, the antenna rod is moved up. At time t1, the antenna rod is raised up to the extreme end in the up movement of the antenna rod. The rod is then stopped by a stopper so that the rotation of the motor is impeded and locked by a damper mechanism, for example. Therefore, after time t1, load current Im increases. However, when the motor is substantially locked, the motor load current Im is limited at Ic, and a current state as indicated by reference numeral 100 is set up.
To realize such current limit, a current limiting transistor is inserted between the motor and the power source, and the transistor is operated in the active region. To this end, a transistor of a relatively large value is used with a sufficient current capacity. Thus, when a large current flow is present, the heat value is high. Also, since the motor current is limited after a relatively large lock current flows a large rotational torque is generated in the motor. The torque is applied to the various types of parts and components existing between the output shaft of the motor and the antenna rod. When the torque acts on components made of, for example, synthetic resin, such as gears, the so-called creep deformation occurs in the gears. This creates a problem of shortening the liftime of the gears.
To solve this, there is used another control means which uses a timer. The timer sets a time t0 long enough for the antenna rod to reach the uppermost position of the rod. When the control enters the phase of current limiting, the motor current is shut off. In this approach, however, a large torque is still present during the period from time t1 to the shut-off of the motor current, as shown in FIG. 15. In this respect, this approach does not provide a complete solution to the above problem.
Additionally, even if the motor current is shut off, with the rotational torque generated when the current is fed to the motor, the elastic energy is left in the damper mechanism. Therefore, after the motor current is shut off, the residual energy provides a force which acts in the opposite direction to that of the motor rotational direction. This force is applied to the gear mechanism. Thus, the above parts and components continuously are under stress for a long time.